1. Field of the Invention
The present invention relates to a bottom resist layer composition useful for a multilayer-resist process used for micropatterning in production process of semiconductor devices etc, and especially to a bottom resist layer composition of a trilayer resist film suitable for exposure with far ultraviolet rays at a wavelength of 300 nm or less like KrF excimer laser light (248 nm) and ArF excimer laser light (193 nm). Furthermore, the present invention also relates to a patterning process for forming a pattern on a substrate with lithography using the composition.
2. Description of the Related Art
It has been needed to make a finer pattern rule along with a tendency in which integration and speed of LSI have become higher in recent years. And in lithography using optical exposure which is used as a general technique at present, resolution has almost reached the inherent limit derived from a wavelength of a light source.
Optical exposure has been widely used using g line (436 nm) or i line (365 nm) of a mercury-vapor lamp as a light source for lithography when a resist pattern is formed. It has been considered that a method of using an exposure light with a shorter wavelength is effective as a means for achieving a further finer pattern. For this reason, for example, KrF excimer laser at a shorter wavelength of 248 nm has been used as an exposure light source instead of i line (365 nm), for mass-production process of a 64 M bit DRAM processing method. However, a light source at far shorter wavelength is needed for manufacture of DRAM with an integration of 1 G or more which needs a still finer processing technique (for example, a processing dimension is 0.13 μm or less), and lithography using ArF excimer laser (193 nm) has been especially examined.
On the other hand, it has been known so far that a multilayer-resist process such as a bilayer resist process or a trilayer resist process is excellent in order to form a pattern with a high aspect ratio on a nonplanar substrate.
Especially, it is supposed that it is preferable to use a high-molecular silicone compound having a hydrophilic group, such as a hydroxy group, a carboxyl group, etc. as a base resin of a top resist layer composition, in order to develop a bilayer resist film with a general alkaline developer in a bilayer resist process.
As the high-molecular silicone compound, there have been proposed for KrF excimer lasers a silicone chemically amplified positive-resist composition in which polyhydroxy benzyl silsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are protected by a t-Boc group is used as a base resin, and which is combined with an acid generator (for example, see Japanese Patent Application Laid-open (KOKAI) No. 6-118651 and SPIE vol. 1925 (1993) p 377). Moreover, there have been proposed for ArF excimer lasers a positive resist in which silsesquioxane that a cyclohexyl carboxylic acid is substituted with an acid labile group is used as a base resin (for example, see Japanese Patent Application Laid-open (KOKAI) No. 10-324748, Japanese Patent Application Laid-open (KOKAI) No. 11-302382, and SPIE vol. 3333 (1998) p 62). Furthermore, there has been proposed for F2 laser a positive resist in which silsesquioxane having a hexafluoro isopropanol as a soluble group is used as a base resin (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2002-55456).
These high-molecular silicone compounds contain poly silsesquioxane containing a ladder structure formed by the condensation polymerization of a trialkoxy silane or a tri halogenated silane in a main chain.
As a base polymer for resist in which silicon is suspended from a side chain, (meth)acrylate polymer containing silicon is proposed (for example, see Japanese Patent Application Laid-open (KOKAI) No. 9-110938 and J. Photopolymer Sci. and Technol. Vol. 9 No. 3 (1996) p 435-446).
Examples of a bottom resist layer used for a bilayer-resist process may preferably include a hydrocarbon compound which can be etched with oxygen gas. It is also desirable for the bottom resist layer to have a high etching resistance, since the layer is further used as a mask in the case of etching a substrate under the layer. When etching of the bottom resist layer using a top resist layer as a mask is conducted according to oxygen gas etching, it is preferable that the bottom resist layer consists of only hydrocarbons which do not contain a silicon atom. Moreover, in order to improve a line width controllability of the top resist layer containing silicon atoms and to reduce irregularity on a pattern side wall and collapse of a pattern due to a stationary wave, it is preferable that the bottom resist layer also has a function as an antireflection film. Specifically, it is desirable that a reflectivity from the bottom resist layer to the top resist layer can be suppressed to 1% or less.
Then, the reflectivity can be suppressed to 1% or less by applying a composition having an optimal refractive index n value which is a real part of refractive index, and an optimal extinction coefficient k value which is an imaginary part of refractive index with a suitable thickness.
Here results of calculation of reflectivity of a substrate while a thickness of a bottom resist layer was changed in a range of 0-500 nm are shown in the FIG. 1 and FIG. 2. It was premised that an exposure wavelength was 193 nm, n value of a top resist layer was 1.74 and k value of that was 0.02.
FIG. 1 is a graph which shows a fluctuation of reflectivity of a substrate while k value of a bottom resist layer was fixed at 0.3, a vertical axis signifies n value of that, a horizontal axis signifies a thickness of the bottom resist layer, the n value was changed in a range of 1.0-2.0, and the thickness was changed in a range of 0-500 nm. In FIG. 1, in the case of assuming a bottom resist layer with a thickness of 300 nm or more for a bilayer resist process, it is found that an optimum value to suppress reflectivity to 1% or less exists in a range of 1.6-1.9 of a refractive index (n value) which is as much as or a little higher than that of a top resist layer.
FIG. 2 is a graph which shows a fluctuation of reflectivity of a substrate while n value of a bottom resist layer was fixed at 1.5, a vertical axis signifies k value of that, a horizontal axis signifies a thickness of the layer, the k value was changed in a range of 0-0.8, and the thickness was changed in a range of 0-500 nm. In FIG. 2, in the case of assuming a bottom resist layer with a thickness of 300 nm or more for a bilayer resist process, it is found that reflectivity can be suppressed to 1% or less when k value is in a range of 0.24-0.15. On the other hand, an optimum k value of antireflection film used with a thin thickness of about 40 nm for a single layer resist process is 0.4-0.5, which is different from an optimum k value of a bottom resist layer with a thickness of 300 nm or more for a bilayer resist process. As described above, it has been revealed that a bottom resist layer with lower k value, namely, with higher transparency is needed in a bilayer resist process.
Then, a copolymer of a polyhydroxy styrene and an acrylate has been examined as a bottom resist layer composition for a wavelength of 193 nm (for example, see SPIE Vol. 4345 (2001) p 50). Polyhydroxy styrene has a very strong absorption at a wavelength of 193 nm, and the k value of itself is high of about 0.6. Then, the k value is adjusted to about 0.25 by carrying out copolymerization with acrylate of which k value is almost 0.
However, etching resistance of acrylate is low during etching of a substrate, compared with that of polyhydoroxystyrene, and it is indispensable to copolymerize acrylate at a significant ratio in order to lower the k value. As a result, etching resistance during etching of a substrate is significantly lowered. The etching resistance influences not only an etch rate but generation of surface roughness after etching. The increase of surface roughness after etching becomes serious due to copolymerization of acrylate.
Then, it has been proposed to use a naphthalene ring which is one of those having a higher transparency at a wavelength of 193 nm than a benzene ring and high etching resistance. For example, there has been proposed a bottom resist layer which has a naphthalene ring or an anthracene ring (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2002-14474). However, k value of a naphthol copolycondensation novolac resin and a polyvinyl naphthalene resin is between 0.3 and 0.4, and does not reach a desired transparency of 0.1 to 0.3, and thus it is necessary to raise a transparency further in order to obtain a desired antireflection effect. Moreover, n value at a wavelength of 193 nm of a naphthol copolycondensation novolac resin and a polyvinyl naphthalene resin is low, and it is 1.4 in a naphthol copolycondensation novolac resin, and is 1.2 in a polyvinyl naphthalene resin according to measurement by the present inventors, and which do not reach the desired range. Furthermore, although an acenaphthylene polymer is proposed (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2001-40293 and Japanese Patent Application Laid-open (KOKAI) No. 2002-214777), n value at a wavelength of 193 nm is lower than that at a wavelength of 248 nm, k value is high, and neither n nor k reaches the desired values.
As described above, a bottom resist layer which has high n value, low k value, high transparency, and a high etching resistance is needed in a bilayer resist process.
On the other hand, a trilayer resist process stacking a top resist layer of a single layer resist without containing silicon, an intermediate resist layer containing silicon atoms under the top layer, and a bottom resist layer of an organic layer under the intermediate layer (for example, see J. Vac. Sci. Technol., 16(6), November/December 1979).
In general, a single layer resist which is a top resist layer of a trilayer resist process and does not contain silicon is superior in resolution to a silicon-containing resist which is a top resist layer of a bilayer resist process. Therefore, a single layer resist with high resolution can be used as an exposure imaging layer in a trilayer resist process.
Moreover, a Spin On Glass (SOG) film is used as an intermediate resist layer, and many SOG films have been suggested.
Here an optimum optical constant (n value, k value) to suppress reflection from a substrate in a trilayer resist process is different from that in a bilayer resist process. The purpose to suppress reflection from a substrate as much as possible, in particular, to suppress a reflectivity of a substrate to 1% or less is the same in both a bilayer resist process and a trilayer resist process. However, antireflection effect is given only to a bottom resist layer in a bilayer resist process, it can be given to either an intermediate resist layer or a bottom resist layer, or to both of them in a trilayer resist process.
For example, an intermediate resist layer containing silicon with antireflection effect has been suggested (for example, see U.S. Pat. No. 6,506,497 specification and U.S. Pat. No. 6,420,088 specification).
In general, antireflection effect is higher in a multi-layer antireflection film than in a single-layer antireflection film. Therefore, the multi-layer antireflection film has been widely and industrially used as an antireflection film for optical materials. And, high antireflection effect can be obtained by giving antireflection effect to both an intermediate resist layer and a bottom resist layer in a trilayer resist process. Namely, if an intermediate resist layer containing silicon atoms functions as an antireflection layer in a trilayer resist process, it is not particularly necessary for a bottom resist layer to function as a superior antireflection layer. A high etching resistance during process of a substrate is rather necessary for a bottom resist layer in a trilayer resist process than antireflection effect.
Here FIG. 3 is a graph which shows a fluctuation of reflectivity of a substrate while n value of a bottom resist layer was fixed at 1.5, k value thereof was fixed at 0.6, a thickness thereof was fixed at 500 nm, n value of an intermediate resist layer was fixed at 1.5, k value thereof was changed in a range of 0-0.4, and a thickness thereof was changed in a range of 0-400 nm.
In FIG. 3, it is found that a sufficient antireflection effect to suppress reflectivity of a substrate to 1% or less can be obtained by setting k value of an intermediate resist layer to be low of 0.2 or less and a thickness thereof properly. k value of an intermediate resist layer as an antireflection layer with a thickness of 100 nm or less is generally necessary to be 0.2 or more in order to suppress a reflectivity of a substrate to 1% or less (see FIG. 2). However, because a bottom resist layer can suppress reflection to some extent in a trilayer structure, k value of an intermediate resist layer is optimum in a range of 0.2 or less.
Next, FIG. 4 and FIG. 5 are graphs which show a fluctuation of reflectivity of a substrate while k value of a bottom resist layer was fixed at 0.2 and 0.6 respectively, and a thickness of an intermediate resist layer and a thickness of a bottom resist layer were changed. The bottom resist layer with k value of 0.2 simulates a bottom resist layer optimized for a bilayer resist process, and the bottom resist layer with k value of 0.6 simulates novolac resin or polyhydroxy styrene exposed to a light at a wavelength of 193 nm. Although a thickness of a bottom resist layer fluctuates depending on topography of a substrate, a thickness of an intermediate resist layer is considered not to fluctuate and the intermediate layer can be applied with a prescribed thickness.
Here, when k value of a bottom resist layer is high (k value is 0.6), a reflectivity of a substrate can be suppressed to 1% or less with a thinner thickness of the bottom layer by selecting an optimum thickness of an intermediate resist layer like 50 nm, 110 nm and 170 nm. When k value of a bottom resist layer is 0.2, a thickness of an intermediate resist layer is hardly limited to suppress a reflectivity of a substrate to 1% or less at 250 nm thickness of the bottom layer. From a standpoint of expanding a selective range of a thickness of an intermediate resist layer suppressing reflection from a substrate, 0.2 is preferable to 0.6 as k value of a bottom resist layer. However, as to an etching resistance during etching a substrate of compositions of which k values at a wavelength of 193 nm are 0.2 and 0.6 respectively, the composition with k value of 0.6 is generally higher. Furthermore, a bottom resist layer with k value of 0.6 can suppress a reflectivity of a substrate to 1% or less with an optimum thickness of an intermediate resist layer even when a thickness of the bottom resist layer is thin of 100 nm or less. And a thinner thickness of a resist layer can be realized because of its high etching resistance.
As described above, a bottom resist layer which has a higher etching resistance during etching a substrate and proper k value to be able to suppress a reflectivity of a substrate even when a thickness of an intermediate resist layer is thin is needed in a trilayer resist process.
Furthermore, when a processed layer to be an underlayer of resist layers is a low dielectric constant insulator film comprising porous silica, there is a problem called poisoning that footing profile occurs or scum is generated in a space portion. The poisoning is considered to occur as follows: porous silica adsorbs ammonia in cleaning process of a substrate in which ammonia is used, ammonia is liberated in a resist process, and the ammonia neutralizes acid in resist generated in an exposed area.
Against such a problem, a bottom resist layer that has a high poisoning-resistant effect in a porous silica insulator film after cleaning of a substrate is needed.